DETERMINATION OF MOLECULAR WEIGHT 31
not, slip the graduated tube over the end of the side tube, and
carefully remove the rubber cork so that no water enters the
stem through the capillary. Remove the stopper of the
Hofmann bottle before dropping it in, and at once push in the
cork. Very shortly a stream of air bubbles will ascend the
graduated tube. When, in the course of a minute or two, the
bubbles cease, remove the cork from the apparatus and extin-
guish the burner. The graduated tube is transferred to the
large cylinder of water by closing the open end with the thumb.
Leave the tube in the water with a thermometer beside it
for J hour. Lift the graduated tube, and whilst holding it
by a collar of paper adjust the levels inside and out. Read off
the volume and note the temperature and barometric pressure.
The density is calculated as follows : —
If v is the volume, / the temperature, B the barometric
pressure, and f the vapour tension of water at /°, then the
corrected volume is given by the formula
_xJ^;/) x 273
This multiplied by 0*00009, the weight of i c.c. of hydrogen,
gives the weight of hydrogen occupying the same volume as
the vaporised substance, from which the density A= — * is
Example. — The following result was obtained with ether :
O"ii46 gram of ether gave 36*3 c.c. at 11° and 752 mm. /= 10
mm. at 11°.
36-3 x (752 - TO) x 273 x 0*00009 =
760 x 284
0-00306 0/ 4
Calculated for C4H10O ; A = 37.
If substances of higher boiling-point have to be vaporised,
the water in the outer jacket is replaced by other liquids of
correspondingly higher boiling-point, such as xylene, b.p. 140°,
aniline, b.p. 182°, ethyl benzoate, b.p. 211°, amyl benzoate, b.p.
260°, diphenylamine, b.p. 310°, &c. A Lothar Meyer air-bath